A phase locked loop (PLL) is used to set a frequency that is produced by an oscillator such that it corresponds to a reference frequency which is produced by a reference oscillator. The matching must be sufficiently accurate so that the phase shift between the two frequencies does not drift away.
The fundamental design of a phase locked loop as is known from the prior art is shown in FIG. 1. A signal that is produced by a reference oscillator 1 at a reference frequency Fref is passed to a 1/R divider, which divides the reference frequency Fref by a divisor R, and produces at the output a signal at the frequency Fref′, which has been divided by R. The signal at the frequency Fref′ is compared with a signal at the frequency FVCO′ by means of a phase detector 3. For this purpose, the two signals are passed to the phase detector inputs 3.1 and 3.2 of the phase detector 3. At its output 3.3, the phase detector 3 produces an output voltage, which is determined from the phase shift between the signal at the frequency FVCO′, which in the following text is also referred to as the slave signal, and the reference signal at the frequency Fref′. The phase detector 3 is followed by a charge pump 4, so that a charge pump current Icp can be produced at the output 4.2 of the charge pump 4, by means of the charge pump 4, via a control input 4.1 of the charge pump 4 and on the basis of the output voltage that is produced by the phase detector 3. The charge pump current Icp is passed to the input 5.1 of a loop filter 5. The voltage Vtune which is produced at the output 5.2 of the loop filter 5, and which is also referred to as the tuning voltage in the following text, is passed to the input 6.1 of a voltage controlled oscillator 6, in order to set the output frequency FVCO of the voltage controlled oscillator 6. The voltage controlled oscillator 6 is often also referred to as a VCO.
In the situation where the phase locked loop PLL is used in a transmitter, the output 6.2 of the voltage controlled oscillator 6 may be followed by a power amplifier 8 in order to amplify the signal and to pass it to an antenna 9. The output signal from the voltage controlled oscillator 6 at the frequency FVCO is passed via a feedback path to a 1/N divider 7, which divides the frequency FVCO down to a frequency FVCO′ which has been divided by the divisor N, and, as mentioned, passes this to the input 3.2 of the phase detector 3.
If the frequency FVCO differs from the reference frequency Fref, the phase shift increases in proportion to the time. The control error in the closed control loop in consequence rises (even if the control gain is finite) until the two frequencies Fref′ and FVCO′ match exactly. The remaining frequency control error thus tends to zero.
The fundamental design of the phase locked loop that is shown in FIG. 1 may, for example, be used in a frequency synthesizer.
In this case, a low-frequency, low-noise reference oscillator is required first of all as a reference oscillator 1 in order to produce a high-frequency carrier frequency with as little noise as possible. The reference frequency Fref which is produced by this is divided by the 1/R divider 2 (which is referred to in the following text as a reference divider) down to a lower frequency Fref′, the so-called comparison frequency. The high-frequency output frequency FVCO from the voltage controlled oscillator 6 is divided by the 1/N divider down to the lower frequency FVCO′. The phase difference between the two frequencies Fref′ and FVCO′ is determined by the phase detector 3, and is converted to a duty cycle. A pulse-width-modulated signal is then produced at the output 3.3 of the phase detector 3. The charge pump 4 evaluates the duty cycle of the pulse-width-modulated signal, and converts the pulse-width-modulated signal in conjunction with the loop filter 5 to the control voltage Vtune, which then controls the voltage controlled oscillator 6.
The loop filter 5 may be in the form of an active or of a passive loop filter. Furthermore, depending on the required technical constraints, the loop filter 5 may be in the form of an integrating loop filter or a non-integrating loop filter. In the situation where the loop filter 5 is in the form of a non-integrating loop filter, the control difference between the two frequencies Fref′ and FVCO′ is just regulated at zero. However, there may still be a phase control error. If the phase shift is likewise intended to be minimized, it is advantageous for the loop filter 5 to be in the form of an integrating filter.
The embodiment of the 1/N divider 7 may include the use of a fixed, dual or multi-modulus high-frequency divider.
FIG. 2 shows the simplified form of a phase locked loop, for example of the phase locked loop shown in FIG. 1, as a linear model. The nominal variable Θi is applied to the positive input of the subtraction unit 25. The controller output variable Θ0/N that has been divided by the divisor N is applied to the negative input of the subtraction unit 25. The control difference which results at the output of the subtraction unit is passed to the control path, which is formed from the phase detector and the charge pump, represented by the block 21, the loop filter with the transfer function ZF(s), represented as the block 22, and the voltage controlled oscillator, represented as the block 23.
The transfer function G(s) for the situation where the control loop is not closed can be derived from this as:
                                                                        G                ⁡                                  (                  s                  )                                            =                                                Θ                  0                                                                      Θ                    i                                    -                                      Θ                    0                                                                                                                          =                                                                    K                    p                                    ⁢                                                            Z                      F                                        ⁡                                          (                      s                      )                                                        ⁢                                                                          ⁢                                      K                    0                                                  sN                                                                                        =                                                                    K                    VCO                                    ⁢                                                                          ⁢                                                            I                      cp                                        ⁡                                          (                                              1                        +                                                  s                          ⁢                                                                                                          ⁢                                                      C                            1                                                    ⁢                                                      R                            1                                                                                              )                                                                                                            Ns                    2                                    ⁡                                      (                                                                  C                        1                                            +                                              C                        2                                            +                                              s                        ⁢                                                                                                  ⁢                                                  C                          1                                                ⁢                                                  C                          2                                                ⁢                                                  R                          1                                                                                      )                                                                                                          Equation        ⁢                                  ⁢        1            where    ZF(s)=the transfer function of the loop filter,    KVCO=the gradient of the voltage controlled oscillator,    Icp=the charge pump current    C1=a first capacitance of the loop filter,    R1=a first resistance of the loop filter, and    C2=a second capacitance of the loop filter.
In a manner corresponding to this, FIG. 3 shows the transfer function G(jω) for the open control loop. For this purpose, the circular frequency ω is plotted logarithmically on the x-axis of the diagram. The magnitude of the transfer function G(jω) is plotted, likewise logarithmically, on the y-axis. The profile of the transfer function G(jω) in the left-hand area of the diagram, which is annotated with the reference symbol 31, is:
                                                    G            ⁡                          (                              j                ⁢                                                                  ⁢                ω                            )                                                ∝                              1                          ω              2                                ⁢                                          ⁢                      (                                          -                40                            ⁢                                                          ⁢              dB              ⁢                              /                            ⁢              decade                        )                                              Equation        ⁢                                  ⁢        2            
The profile of the transfer function G(jω) in the central area, which is identified by the reference symbol 32, is:
                                                    G            ⁡                          (                              j                ⁢                                                                  ⁢                ω                            )                                                ∝                              1            ω                    ⁢                                          ⁢                      (                                          -                20                            ⁢                                                          ⁢              dB              ⁢                              /                            ⁢              decade                        )                                              Equation        ⁢                                  ⁢        3            
For the circular frequency ω1:
                              ω          1                =                  1                                    R              1                        ⁢                          C              1                                                          Equation        ⁢                                  ⁢        4            
For the circular frequency ωn:
                              ω          n                =                                                            K                VCO                            ⁢                              I                cp                                                    N              ⁡                              (                                                      C                    1                                    +                                      C                    2                                                  )                                                                        Equation        ⁢                                  ⁢        5            
For the circular frequency ωL:
                              ω          L                =                              ω            n            2                                ω            1                                              Equation        ⁢                                  ⁢        6            
And for the circular frequency ω2:
                              ω          2                =                              ω            1                    ⁡                      (                          1              +                                                C                  1                                                  C                  2                                                      )                                              Equation        ⁢                                  ⁢        7            
A simple passive filter has been assumed as the loop filter 5 or 22 with the transfer function ZF(s), in which a second capacitor C2 is connected in parallel with a first resistor R1 and a first capacitor C1 that are connected in series. The corresponding frequencies ω1, ωn, ωL and ω2 are shown in FIG. 3. The statements that have been made so far relating to the phase locked loop have been based on the assumption that the divisor N is constant.
The loop gain OpenLoopGain of the open control loop is thus:    OpenLoopGain=f(KVCO, Icp, LF(Cx, Rx)),where LF(Cx, Rx) represents the characteristics of the loop filter as a function of the capacitances Cx and of the resistances Rx.
If, in addition, a damping factor D is introduced in the negative feedback path of the phase locked loop, the transfer function H(jω) for the closed control loop situation can be calculated as follows:
                    D        =                              ω            n                                2            ⁢                                                  ⁢                          ω              1                                                          Equation        ⁢                                  ⁢        8                                          H          ⁡                      (            s            )                          =                              Θ            0                                Θ            i                                              Equation        ⁢                                  ⁢        9                                                          ⁢                  =                      1                          1              +                              1                                  G                  ⁡                                      (                    s                    )                                                                                                                                                                    ⁢                  =                                                    ω                n                2                            +                              2                ⁢                D                ⁢                                                                  ⁢                                  ω                  n                                ⁢                s                                                                    ω                n                2                            +                              2                ⁢                D                ⁢                                                                  ⁢                                  ω                  n                                ⁢                s                            +                                                s                  2                                ⁡                                  (                                      1                    +                                          s                                              ω                        2                                                                              )                                                                                                                                                  ⁢                  =                                                                      ω                  L                  2                                                  2                  ⁢                                      D                    2                                                              +                                                ω                  L                                ⁢                s                                                                                      ω                  L                  2                                                  2                  ⁢                                      D                    2                                                              +                                                ω                  L                                ⁢                s                            +                                                s                  2                                ⁡                                  (                                      1                    +                                          s                                              ω                        2                                                                              )                                                                                                    
The diagram in FIG. 4, in which the circular frequency is plotted logarithmically on the X-axis and in which the transfer function H(jω) for the closed control loop is plotted, likewise logarithmically, on the Y-axis, shows a good approximation to the spectral profile of the locked-in phase locked loop in the area to the right of the carrier. The loop bandwidth is denoted ωx, and is calculated to be:
                              ω          x                =                                            2              ⁢                                                          ⁢              D              ⁢                                                          ⁢                              ω                n                            ⁢                              ω                2                                              =                                    ω              n                        ⁢                                          1                +                                                      C                    1                                                        C                    2                                                                                                          Equation        ⁢                                  ⁢        10            
If the capacitance C2 is equal to zero, that is to say the capacitance C2 does not exist, so that the loop filter 5 comprises just the first resistor R1 and the first capacitance C1 connected in series, this results in the profile which is identified by the reference symbol 42 and is illustrated by dashed lines in FIG. 4. The profile which is identified by the reference symbol 42 has a drop of −20 dB/decade. If the capacitance C2 is greater than zero, this results in the profile of the transfer function H(jω) denoted by the reference symbol 41 in FIG. 4, which then has a drop of −40 dB/decade.
Particularly in the case of frequency synthesizers for wire-free radio systems, it is essential to monitor the loop bandwidth ωx as accurately as possible. This is because the stability of the control loop, the speed of any sudden frequency change and the total phase error of the phase locked loop, and hence the purity of the modulation which interacts with the peak of the phase noise (jitter), are influenced directly or indirectly by the loop bandwidth ωx.
The gradient KVCO of the voltage controlled oscillator, the charge pump current Icp of the charge pump and the components of the loop filter influence the loop bandwidth ωx. Since the gradient KVCO of the voltage controlled oscillator, the charge pump current and the resistors and capacitors in the loop filter do not always have an exactly defined value owing to manufacturing tolerances, fluctuations in the supply voltage, temperature fluctuations, etc, and are thus subject to variations, these influencing factors result in a deterioration in the loop bandwidth ωx, which leads to a decreased in the loop bandwidth ωx. All the discrepancies in the loop bandwidth ωx thus have considerable negative effects on the quality, and thus also on the yield, of the fabricated circuits.
The document U.S. Pat. No. 5,786,733 specifies a phase locked loop in which frequency discrepancies can be detected. The oscillator is preceded by a variable gain amplifier, which is driven as a function of the frequency discrepancies over time.